Cellular senescence is considered a major tumour-suppressor mechanism in mammals, and many oncogenic insults, such as the activation of the ras proto-oncogene, trigger initiation of the senescence programme. Although it was shown that activation of the senescence programme involves the up-regulation of cell-cycle regulators such as the inhibitors of cyclin-dependent kinases p16INK4A and p21CIP-1, the mechanisms underlying the senescence response remain to be resolved. In the case of stress-induced premature senescence, reactive oxygen species are considered important intermediates contributing to the phenotype. Moreover, distinct alterations of the cellular carbohydrate metabolism are known to contribute to oncogenic transformation, as is best documented for the phenomenon of aerobic glycolysis. These findings suggest that metabolic alterations are involved in tumourigenesis and tumour suppression; however, little is known about the metabolic pathways that contribute to these processes. Using the human fibroblast model of in vitro senescence, we analysed age-dependent changes in the cellular carbohydrate metabolism. Here we show that senescent fibroblasts enter into a metabolic imbalance, associated with a strong reduction in the levels of ribonucleotide triphosphates, including ATP, which are required for nucleotide biosynthesis and hence proliferation. ATP depletion in senescent fibroblasts is due to dysregulation of glycolytic enzymes, and finally leads to a drastic increase in cellular AMP, which is shown here to induce premature senescence. These results suggest that metabolic regulation plays an important role during cellular senescence and hence tumour suppression.

Limitation of lifespan in replicative senescence is related to oxidative stress, which is probably both the cause and consequence of impaired mitochondrial respiratory function. The respiration of senescent human diploid fibroblasts was analysed by high-resolution respirometry. To rule out cell-cycle effects, proliferating and growth-arrested young fibroblasts were used as controls. Uncoupled respiration, as normalized to citrate synthase activity, remained unchanged, reflecting a constant capacity of the respiratory chain. Oligomycin-inhibited respiration, however, was significantly increased in mitochondria of senescent cells, indicating a lower coupling of electron transport with phosphorylation. In contrast, growth-arrested young fibroblasts exhibited a higher coupling state compared with proliferating controls. In intact cells, partial uncoupling may lead to either decreased oxidative ATP production or a compensatory increase in routine respiration. To distinguish between these alternatives, we subtracted oligomycin-inhibited respiration from routine respiration, which allowed us to determine the part of respiratory activity coupled with ATP production. Despite substantial differences in the respiratory control ratio, ranging from 4 to 11 in the different experimental groups, a fixed proportion of respiratory capacity was maintained for coupled oxidative phosphorylation in all the experimental groups. This finding indicates that the senescent cells fully compensate for increased proton leakage by enhanced electron-transport activity in the routine state. These results provide a new insight into age-associated defects in mitochondrial function and compensatory mechanisms in intact cells.

Escalation of drug use, a hallmark of drug dependence, has traditionally been interpreted as reflecting the development of tolerance to the drug’s effects. However, on the basis of animal behavioral data, several groups have recently proposed alternative explanations, i.e. that such an escalation of drug use might not be based on (1) tolerance, but rather be indicative of (2) sensitization to the drug’s reinforcing effect, (3) reward allostasis, (4) an increase in the incentive salience of drug-associated stimuli, (5) an increase in the reinforcing strength of the drug reinforcer relative to alternative reinforcers, or (6) habit formation. From the pharmacological perspective, models 1–3 allow predictions about the change in the shape of drug dose-effect curves that are based on mathematically defined models governing receptor-ligand interaction and signal transduction. These predictions are tested in the present review, which also describes the other currently championed models for drug use escalation and other components of apparent ‘reinforcement’ (in its original meaning, like ‘tolerance’ or ‘sensitization’, a purely descriptive term). It evaluates the animal experimental approaches employed to support or prove the existence of each of the models and reinforcement components, and recapitulates the clinical evidence, which strongly suggests that escalation of drug use is predominantly based on an increase in the frequency of intoxication events rather than an increase in the dose taken at each intoxication event. Two apparent discrepancies in animal experiments are that (a) sensitization to overall reinforcement has been found more often for psychostimulants than for opioids, and that (b) tolerance to the reinforcing and other effects has been observed more often for opioids than for cocaine. These discrepancies are resolved by the finding that cocaine levels seem to be more tightly regulated at submaximum reinforcing levels than opioid levels are. Consequently, animals self-administering opioids are more likely to expose themselves to higher above-threshold doses than animals self-administering psychostimulants, rendering the development of tolerance to opioids more likely than tolerance to psychostimulants. The review concludes by making suggestions on how to improve the current behavioral experimental approaches.

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